This invention relates to phosphate ester fluids used in transmitting power in hydraulic systems. More specifically it relates to enhancing the anti-erosion properties of such fluids.
Functional fluids are used in a wide variety of industrial applications. For example they are used as the power transmitting medium in hydraulic systems, such as aircraft hydraulic systems.
Functional fluids intended for use in aircraft hydraulic systems must meet stringent performance criteria such as thermal stability, fire resistance, low susceptibility to viscosity changes over a wide range of temperatures, good hydrolytic stability, elastomer compatibility and good lubricity.
Organic phosphate ester fluids have been recognized as a preferred fluid for use as a functional fluid such as in hydraulic fluids. Indeed, in present commercial aircraft hydraulic fluids phosphate esters are among the most commonly used base stocks.
As with other functional fluids, organic phosphate ester based fluids require the incorporation of various additives to enhance the performance of the fluid. For example, experience has shown that orifices in the servo control valves of aircraft hydraulic systems are subject to erosion which is attributed to streaming current induced by fluid flow. Valve orifice erosion, if extensive, can greatly impair the functioning of the valve as a precise control mechanism. Therefore various additives have been used in functional fluids as erosion inhibitors. Nonetheless, there remains a need for increased choice of useful erosion inhibitors, especially for improved erosion inhibitors.
One object of the present invention is to provide phosphate ester based aircraft hydraulic fluids with enhanced anti-erosion properties.
Briefly stated a phosphate ester functional fluid, especially a hydraulic fluid, is enhanced by incorporating in the fluid an erosion inhibiting an erosion inhibiting amount of an additive or mixture thereof having the formula
RfCH2CH2SO3M 
where Rf=F(CH2CF2)y, y=1 to about 9 and M is an alkali metal selected from lithium, sodium, potassium, rubidium or ceasium. M can also be a quaternary ammonium cation having following formula RRxe2x80x2Rxe2x80x3Rxe2x80x2xe2x80x3N⊕ wherein R, Rxe2x80x2, Rxe2x80x3, Rxe2x80x2xe2x80x3 are independently hydrogen, hydrocarbyl groups of from 1 to 30 carbon atoms and oxygen containing hydrocarbyl groups of from 1 to 25 carbon atoms.